Conventional radar displays consist of circular-shaped cathode ray tubes (CRT) in which a rotating beam is made visible by electron bombardment of a phosphor coating on a CRT screen. Targets can be identified on the screen as the beam rotates in synchronism with a rotating antenna. This type of display is known as a Plan Position Indicator (PPI).
A PPI display has a number of objectionable characteristics. Because of the fact that it relies on the persistence of a phosphor, there is an inherent lack of brightness. Thus, the early tubes could be viewed satisfactorily only under very low levels of ambient lighting, and refreshing of the PPI display occurred only once per revolution of the radar antenna, and it, therefore, was dependent on the radar revolution rate.
In order to overcome these deficiencies and to achieve other advantages, scan converters have been developed to convert the PPI information, which is a function of the radius (r) and the angle (.theta.) of the radial beam from a reference location to TV or computer screen monitors in which the (x) and (y) coordinates of the screen are used to determine the image. Scan converter systems allow for the integration of radar displays and with computer video recording techniques, including multiple color displays, overlapping windows and the capability of adding text to the display.
Numerous types of such systems have been developed for providing the conversion of the (r,.theta.) information into the (x,y) information. The great majority of these relied on relatively complex hardware-dominated systems for providing the scan conversion. In the past such complex hardware systems were required to achieve the fast speed needed to process the real-time information being received from the radar return.
Software algorithms for radar coordinate digital scan conversion have been developed, as shown in U.S. Pat. No. 4,697,185 entitled "Algorithm for Radar Coordinate Conversion and Digital Scan Converters," issued Sep. 29, 1987 to David M. Thomas et al., and U.S. Pat. No. 4,931,801 entitled "Method and Apparatus to Scan Convert Radar Video to Television Outputs, " issued Jun. 5, 1990 to William R. Hancock. These algorithms were joined with specialized hardware to provide the desired (r,.theta.) to (x,y) scan conversion.
In the Thomas et al. patent it was noted that near the center or origin of a PPI display, the azimuthal resolution of the radar is greater than the resolution of the display, and, therefore, a number of (r,.theta.) points must be matched to the same (x,y) point. At long ranges in a PPI display, however, the radar resolution will often be less than that of the display. This results in a number of open areas in the display which have to be filled in. At intermediate ranges, the resolution of the radar and the display are approximately equal, and there may be a one-to-one mapping between the two coordinate systems.
In the Thomas et al. patent, look-up tables are utilized to hold sin and cos values to update the x and y values to the next consecutive coordinate of x and y values by adding a sin value to the x coordinate and a cos value to the y coordinate. In the Hancock patent look-up tables were also employed to control intensities of the display pixels. Look-up tables have also been employed in graphic displays to control colors of the image displayed.